Contributions of the kidneys and intestines to water conservation, and plasma levels of antidiuretic hormone, during dehydration in house sparrows (Passer domesticus)

Water restriction increases oxidation of endogenous amino acids in house sparrows (Passer domesticus)

Animals can cope with dehydration in a myriad of ways, both behaviorally and physiologically. The oxidation of protein produces more metabolic water per kilojoule than that of fat or carbohydrate, and it is well established that birds increase protein catabolism in response to high rates of water loss. However, the fate of amino acids mobilized in response to water restriction has not been explicitly determined. While protein catabolism releases bound water, we hypothesized that water-restricted birds would also oxidize the resulting amino acids, producing additional water as a product of oxidative phosphorylation. To test this, we fed captive house sparrows (Passer domesticus) 13C-labeled leucine for 9 weeks to label endogenous proteins. We conducted weekly trials during which we measured the physiological response to water restriction as changes in lean mass, fat mass, metabolism and the enrichment of 13C in exhaled CO2 (δ13Cbreath). If water-restricted birds catabolized proteins and oxidized the resulting amino acids, we expected to simultaneously observe greater lean mass loss and elevated δ13Cbreath relative to control birds. We found that water-restricted birds catabolized more lean tissue and also had enriched δ13Cbreath in response to water restriction, supporting our hypothesis. δ13Cbreath, however, varied with metabolic rate and the length of the water restriction period, suggesting that birds may spare protein when water balance can be achieved using other physiological strategies.

Intestinal electrogenic sodium-dependent glucose absorption in tilapia and trout reveal species differences in SLC5A-associated kinetic segmental segregation

Electrogenic sodium-dependent glucose transport along the length of the intestine was compared between the omnivorous Nile tilapia ( Oreochromis niloticus) and the carnivorous rainbow trout ( Oncorhynchus mykiss) in Ussing chambers. In tilapia, a high-affinity, high-capacity kinetic system accounted for the transport throughout the proximal intestine, midintestine, and hindgut segments. Similar dapagliflozin and phloridzin dihydrate inhibition across all segments support this homogenous high-affinity, high-capacity system throughout the tilapia intestine. Genomic and gene expression analysis supported findings by identifying 10 of the known 12 SLC5A family members, with homogeneous expression throughout the segments with dominant expression of sodium-glucose cotransporter 1 (SGLT1; SLC5A1) and sodium-myoinositol cotransporter 2 (SMIT2; SLC5A11). In contrast, trout's electrogenic sodium-dependent glucose absorption was 20-35 times lower and segregated into three significantly different kinetic systems found in different anatomical segments: a high-affinity, low-capacity system in the pyloric ceca; a super-high-affinity, low-capacity system in the midgut; and a low-affinity, low-capacity system in the hindgut. Genomic and gene expression analysis found 5 of the known 12 SLC5A family members with dominant expression of SGLT1 ( SLC5A1), sodium-glucose cotransporter 2 (SGLT2; SLC5A2), and SMIT2 ( SLC5A11) in the pyloric ceca, and only SGLT1 ( SLC5A1) in the midgut, accounting for differences in kinetics between the two. The hindgut presented a low-affinity, low-capacity system partially attributed to a decrease in SGLT1 ( SLC5A1). Overall, the omnivorous tilapia had a higher electrogenic glucose absorption than the carnivorous trout, represented with different kinetic systems and a greater expression and number of SLC5A orthologs. Fish differ from mammals, having hindgut electrogenic glucose absorption and segment specific transport kinetics.

Measurement of glomerular filtration rate during flight in a migratory bird using a single bolus injection of FITC-inulin

During migration, passerine birds typically complete a series of multi-hour flights, each followed by a period of stopover. During flight, rates of respiratory water loss are high, yet these birds show no signs of dehydration after flights. During stopover, birds become hyperphagic to replenish fat reserves, often consuming food with high water content, such as fruit. Thus migratory birds seem to face an osmoregulatory challenge; they must reduce water losses during flight but retain the ability to excrete large quantities of water while maintaining osmotic balance at stopover. Our goal was to measure glomerular filtration rate (GFR) and fractional water reabsorption (FWR) of a migratory bird in free flight, at rest, and during feeding to assess the role of the kidney in maintaining water balance during migration. We used FITC-inulin and one- and two-phase exponential decay models to first validate a technique and then measure GFR in the Swainson's thrush, a small (∼30 g) songbird. Single-phase exponential decay models and the modified slope intercept method overestimated GFR by 26% compared with two-phase exponential decay models. We found no differences in GFR among fed, resting and flying birds, but FWR was significantly higher in resting and flying birds relative to feeding birds. There was no effect of the rate of respiratory water loss on GFR or FWR in flight. These data support the idea that birds in flight do not dramatically alter GFR but rely on increased FWR to minimize excretory water losses.

House sparrows (Passer domesticus) increase protein catabolism in response to water restriction

Birds primarily rely on fat for energy during fasting and to fuel energetically demanding activities. Proteins are catabolized supplemental to fat, the function of which in birds remains poorly understood. It has been proposed that birds may increase the catabolism of body protein under dehydrating conditions as a means to maintain water balance, because catabolism of wet protein yields more total metabolic and bound water (0.155·H(2)O(-1)·kJ(-1)) than wet lipids (0.029 g·H(2)O(-1)·kJ(-1)). On the other hand, protein sparing should be important to maintain function of muscles and organs. We used quantitative magnetic resonance body composition analysis and hygrometry to investigate the effect of water restriction on fat and lean mass catabolism during short-term fasting at rest and in response to a metabolic challenge (4-h shivering) in house sparrows (Passer domesticus). Water loss at rest and during shivering was compared with water gains from the catabolism of tissue. At rest, water-restricted birds had significantly greater lean mass loss, higher plasma uric acid concentration, and plasma osmolality than control birds. Endogenous water gains from lean mass catabolism offset losses over the resting period. Water restriction had no effect on lean mass catabolism during shivering, as water gains from fat oxidation appeared sufficient to maintain water balance. These data provide direct evidence supporting the hypothesis that water stress can increase protein catabolism at rest, possibly as a metabolic strategy to offset high rates of evaporative water loss.

Adaptive strategies for post-renal handling of urine in birds

Birds are a diverse vertebrate class in terms of diet and habitat, but they share several common physiological features, including the use of uric acid as the major nitrogenous waste product and the lack of a urinary bladder. Instead, ureteral urine refluxes from the urodeum into the more proximal coprodeum and portions of the hindgut (colon or rectum and ceca). This presents a potential problem in that hyperosmotic ureteral urine in contact with the permeable epithelia of these tissues would counteract renal osmotic work. This review describes and provides examples of different strategies used by avian species to balance renal and post-renal changes in urine composition. The strategies described include: 1. a "reptilian" mode, with moderate renal concentrating ability, but high rates of post-renal salt and water resorption; 2. the "mammalian" strategy, in which the coprodeum effectively functions like a mammalian urinary bladder, preserving the osmotic concentrating work of the kidney; 3. an interaction strategy, in which post-renal transport processes are hormonally regulated in order to optimize renal function under varying conditions of salt or water stress; 4. the salt gland strategy seen in marine or estuarine birds with functional salt glands, in which post-renal transport mechanisms are used to conserve urinary water and to recycle excess NaCl to the nasal salt glands. Finally, we also describe some features of an as-yet unstudied group of birds, the birds of prey. At least some species in this group are relatively good renal concentrators, and would be predicted to have post-renal mechanisms to preserve this work. This new synthesis illustrates the marked diversity of adaptive mechanisms used by avian species to maintain osmotic homeostasis.

Expression and localization of an aquaporin-1 homologue in the avian kidney and lower intestinal tract

In birds, the kidneys and lower intestine function in osmoregulation. A 271-amino acid homologue to aquaporin-1 (AQP-1) was isolated from the kidneys, cecae, proximal and distal rectum, and coprodeum of the lower intestine in the house sparrow (Passer domesticus). This protein has six transmembrane domains connected by two cytoplasmic loops and three extracellular loops. It exhibits 94%, 88%, and 78% homology to AQP-1 sequences of chicken, human and toad, respectively. Many of the highly conserved amino acids that are characteristic of AQP-1 are found in the sparrow sequence. RT-PCR was performed and the presence of AQP-1 mRNA was detected in the kidney and all four regions of the lower intestine. Immunoblots of total protein identified a 28-kDa non-glycosylated AQP-1 band and a 56-kDa glycosylated AQP-1 band in the kidney and all four regions of the lower intestine. Immunohistochemistry demonstrated the presence of the AQP-1 protein within both the renal cortex and medulla. In the lower intestine, the protein was present in the proximal rectum, distal rectum, and in the coprodeum. As AQP-1 functions to allow water movement across mammalian cell membranes, its presence in water-permeable cells in a bird suggests it may have a similar function.

Regulation of the avian kidney by arginine vasotocin

Nonapeptides secreted by the neurohypophysis have important roles in vertebrate cardio-fluid homeostasis. In birds, those peptides include mesotocin (the representative of the neutral, or oxytocin-like, nonapeptide family) and vasotocin (the representative of the basic, or vasopressin-like, hormones). The function of mesotocin is not well defined, but it does appear to have osmoregulatory functions. Vasotocin is considered the primary avian antidiuretic hormone. Receptors for AVT in avian kidney-either on renal vasculature or on the tubules-have yet to be localized or identified. However, AVT quite certainly effects antidiuresis via both vascular and tubular mechanisms. The former entail a reduction in the rate of glomerular filtration, apparently via constriction of afferent arterioles. Evidence for the latter (direct tubular action of AVT) has accumulated in recent years and includes enhanced fractional tubular water reabsorption, activation of second messenger (cAMP) in thick ascending limbs and collecting ducts, and modest AVT-stimulated water permeability in collecting ducts associated with expression of aquaporins. The relative importance of the renal vascular vs. tubular actions in birds likely depend on the dose of the hormone, the physiological condition of the animal, and the species of bird.

Physiological and hormonal control of thermal depression in the tiger snake, Notechis scutatus

Plasma sodium concentrations in field-caught Western tiger snakes, Notechis scutatus, from semi-arid Carnac Island (CI) varied seasonally, with snakes exhibiting significant hypernatraemia during summer and normal concentrations following autumn rain. In contrast, field-caught tiger snakes from a perennial fresh-water swamp (Herdsman Lake, HL) exhibited no significant increase in plasma sodium concentrations during summer. Laboratory-induced hypernatraemia caused thermal depression in both populations; there was a weak negative relationship between plasma sodium concentration and temperature selection that was significant for CI snakes. Hypernatraemia significantly elevated circulating concentrations of the neuropeptide arginine vasotocin (AVT) in both CI and HL snakes. CI snakes injected with a physiological dosage of AVT also evidenced thermal depression. Despite the positive correlation between AVT and both plasma sodium concentration and osmolality for laboratory snakes, field samples from CI snakes indicate that circulating levels of AVT may be influenced more by plasma osmolality than sodium levels. The data suggest that, in CI snakes, chronic dehydration in the field leads to hypernatraemia which may lead to elevated levels of AVT if plasma osmolality also increases. This will in turn invoke a depression in thermal behaviour that may improve the water economy and survival of snakes on semi-arid CI. Although HL snakes do not experience seasonal dehydration, physiological changes away from the stable homeostatic state appear to prompt the same behavioural shifts, illustrating the intrinsic nature of the thermal behaviour in different populations of the same species of snake.

Osmoregulatory response to acute diet change in an avian nectarivore: rapid rehydration following water shortage

Nectarivores may be required to switch between water conservation and water excretion as rapidly as they change food plants in nature. We examined the rehydration response in Whitebellied Sunbirds (Nectarinia talatala) that had been fed a concentrated sucrose diet (2.5 mol/l) for 2 days and then were switched to a diet 10 x less concentrated (0.25 mol/l) on the morning of the third day. We measured water gain as well as cloacal fluid (CF) volume hourly over 12 h, and analysed CF osmolality and calculated osmotic excretion. CF was also assayed for the osmoregulatory hormone aldosterone (ALDO). As in most water-deprived birds, whitebellied sunbirds cope with water shortage when fed a concentrated sugar diet by almost completely ceasing to void CF. Although osmolality of CF is high, volumes are not sufficient to maintain a steady rate of excretion and the birds retain osmolytes. Immediately upon switching to dilute diet, sunbirds produce copious volumes of CF and osmotic excretion is elevated and maintained at high levels over the first 6 h of rehydration. This stabilises by the afternoon at levels expected for hydrated birds. Some 2-3 h after peak osmotic excretion, there is a peak in the discrepancy between water intake and output, mirrored in an increase in ALDO output. These data suggest that excretion of retained osmolytes is undertaken as soon as water is available, with changes in the body fluid composition occurring subsequently. This study vindicates the use of CF to obtain repeated measurements of changes in the osmoregulatory steroid ALDO in small birds.

Comparison of renal and salt gland function in three species of wild ducks

Three processes central to osmoregulation of marine birds were compared in three species of ducks that differ in habitat affinity, diet and saline tolerance. These processes are filtration of Na+ and water from the plasma by the kidneys, their reabsorption along the renal tubules, and secretion by the salt glands. Barrow's goldeneyes Bucephala islandica, the most marine species, have the highest rates for all three processes and only this species can secrete all the infused salt via the salt glands. Rates of all three processes are lower in mallards Anas platyrhynchos, the most freshwater species. Following saline acclimation, mallards could excrete all the infused Na+ by a combined Na+ excretion of the kidneys and salt glands. Canvasbacks Aythya valisineria, despite being more saline tolerant than mallards, are unable to excrete all the infused Na+. They produce a large volume of urine (like mallards) that has a low [Na+] (like goldeneyes). Salt gland secretion Na+ concentration did not differ among the three species, but only goldeneyes secrete at a rate sufficient to eliminate all infused Na+ via the salt glands. Differences in saline tolerance of these ducks species cannot be fully explained by differences in their filtration, reabsorption and secretion of Na+ and water, suggesting that the intestinal tract plays an important role.

Osmoregulation in an avian nectarivore, the whitebellied sunbird Nectarinia talatala: response to extremes of diet concentration

Water intake of nectarivores is intrinsically linked to nectar concentration. Osmoregulation in whitebellied sunbirds Nectarinia talatala (body mass 9.3+/-0.1 g, mean +/- S.D., N=7), was examined by feeding them sucrose solutions, equivalent to extreme diet concentrations (0.07-2.5 mol l(-1) sucrose; 2-65% w/w), with and without supplementary drinking water. Total water gain was 33-515% of body mass daily. Cloacal fluid (CF) volume increased with diet dilution from 0.4% to 309% of body mass while increases in evaporative water loss (obtained by difference) were also recorded. Osmolality of CF demonstrated the largest scope yet recorded for a bird and was significantly correlated with water flux: mean values were 6-460 mosm kg(-1) H(2)O (minimum 3, maximum 1900 mosm kg(-1)). When supplementary water was provided, its consumption by birds fed concentrated diets (2.5 mol l(-1) sucrose) led to a dramatic reduction in CF osmolality, from 461+/-253 to 80+/-119 mosm kg(-1) fluid. Sunbirds maintained energy balance on sucrose diets varying tenfold in concentration, from 0.25 to 2.5 mol l(-1); however, on extremely dilute diets (0.07 and 0.1 mol l(-1) sucrose, lower than natural nectar concentrations) their inability to maintain energy balance was probably due to excess preformed water. Total osmotic excretion and concentrations of Na(+) and K(+) increased with high water fluxes, and are a possible physiological constraint for nectarivorous birds on artificial dilute diets devoid of electrolytes. Even low electrolyte levels in nectars may be adequate to replace these losses, but other physiological limitations to the intake of dilute nectars are increased energetic costs of solute recovery, increased heat loss and interference with digestive processes. Sunbirds therefore deal with sugar solutions spanning the range of nectar concentrations by shutting down water excretion on concentrated diets, or, on dilute diets, by producing extremely dilute CF with some of the lowest solute concentrations recorded.

Clinical chemistry of companion avian species: a review

Birds have evolved alternate physiologic strategies to contend with dehydration, starvation, malnutrition, and reproduction. Basic anatomic and functional differences between birds and mammals impact clinical chemistry values and their evaluation. Interpretation of the results of standard biochemical analyses, including BUN, alanine aminotransferase, aspartate aminotransferase, creatine kinase, gamma glutamyltransferase, bilirubin, ammonia, alkaline phosphatase, cholesterol, bile acids, glucose, albumin, globulins, calcium, phosphorus, prealbumin (transthyretin), fibrinogen, iron, and ferritin, is reviewed and discussed in relation to these physiological differences. The use and interpretation of alternative analytes appropriate for avian species, such as uric acid, biliverdin, glutamate dehydrogenase, and galactose clearance, also are reviewed. Normal avian urine and appropriate use of urinalysis, an integral part of laboratory diagnosis in mammalian species that frequently is omitted from avian diagnostic protocols, is discussed.

Luminal morphology of the avian lower intestine: evidence supporting the importance of retrograde peristalsis for water conservation

Tissue from the lower intestine of two species of sparrow, the house sparrow Passer domesticus and savannah sparrow Passerculus sandwichensis was sectioned in an unbiased manner and examined quantitatively using stereology. The tissue was processed for light microscopy, and scanning and transmission electron microscopy to examine the extent to which microvilli enhanced the epithelial surface area of the cecae, rectum, and coprodeum. Parameters measured included individual microvillus surface area, microvilli packing density, and absolute surface area. In both species, the average surface area, packing density, and absolute surface area of microvilli decreased distally along the rectum and coprodeum. All three measured variables were not statistically significant (P > 0.05) between species. Surface area amplification on the cecae due to microvilli was low, and approximated values equivalent to distal regions of the rectum and coprodeum. In the cecae, microvilli within the savannah sparrow had a significantly higher (P < 0.05) individual surface area, packing density, and absolute surface area than in the house sparrow. The functional implications of a change in microvilli population are discussed in relation to retrograde peristalsis within the lower intestine of birds.

Renal response to dietary protein in the house sparrow Passer domesticus

Many birds switch seasonally or during ontogeny between diets of varying protein content. In mammals, high-protein diets induce hypertrophy of the kidney in general and of the thick ascending limbs (TAL) in particular, along with increases in glomerular filtration rate (GFR) and urine flow. A hypothesis to explain these phenomena is that the TAL become increasingly sensitive to peptide hormones (glucagon and antidiuretic hormone [ADH]) released in response to protein feeding; the consequent enhancement of ion reabsorption dilutes urine reaching the macula densa, thereby suppressing tubulo-glomerular feedback (TGF) and causing a rise in GFR. Avian kidneys possess most of the elements involved in this mechanism, including loops of Henle with TAL, sensitivity of TAL to ADH (arginine vasotocin [AVT] in birds), and the elements of TGF. We therefore hypothesized that switching from a low-protein to a high-protein diet would induce responses in birds similar to those found in mammals. We tested this hypothesis by feeding house sparrows, Passer domesticus, isocaloric diets containing either 8% or 30% protein. Birds on high-protein food had larger renal medullae, both in mass and in TAL diameter, but no increase in whole-kidney mass. Urine flow was approximately doubled on high-protein food, but there was no change in GFR. We were not able to detect an increased sensitivity of AVT-induced adenylyl cyclase activity in TAL from high-protein animals, and responsiveness to glucagon was higher in TAL from birds eating low-protein food. We are unable to conclude that a suppression of TGF is responsible for the rise in urine flow in birds eating high-protein foods, and the mechanisms behind the medullary hypertrophy and the diuresis remain to be fully explored.

Renal compensation for cecal loss in Gambel's quail (Callipepla gambelii)

Some studies have implicated the avian digestive cecae as important sites of water and solute reclamation working in concert with the lower intestine and the kidneys as part of an integrated osmoregulatory system. In Gambel's quail (Callipepla gambelii), we studied compensatory adjustments in renal function on days 6-7 and 16-17 following ligation of cecae. Plasma osmolality (Posm) varied significantly between groups with sham-operated birds (Cs), with an average (Posm) of 348 mOsm/kg H2O and quail with ligated cecae (Cx) having a (Posm) of 355 mOsm/kg H2O. We detected no change in the rate of glomerular filtration (GFR) between experimental and control groups either shortly after cecectomy or after 16-17 d following surgery. Regression analysis of GFR and urine flow rate (V) showed that Cx birds had a significantly lower V at a given GFR than did controls, evidence that Cx quail absorbed more fluid in their renal tubules. Increased fluid reabsorption was apparently driven by an enhanced reabsorption of sodium. Indeed, sodium excretion was lower in Cx quail as compared to sham-operated birds. On days 6-7, Cx quail drank more water than Cs birds, but by days 16-17 drinking rates were similar. At the end of the experiments, Cx quail showed a proliferation of microvilli along the apical membrane of the rectum, an adjustment consistent with the idea that the rectum alters its absorption capacity to adjust for the loss of cecal function.

Entry of nephrons into the collecting duct network of the avian kidney: a comparison of chickens and desert quail

The avian kidney contains a population of nephrons with and without loops of Henle. How the collecting ducts of this heterogeneous population of nephrons merge to exit as single ducts from the medullary cones has been uncertain. The results of this study show that the collecting duct tree begins with the coalescence of the distal tubules of pairs of loopless nephrons. These primary collecting ducts receive output from only loopless nephrons. Primary collecting ducts fuse in pairs and become secondary collecting ducts. They receive the distal tubules of transition nephrons. Pairs of secondary collecting ducts fuse and become tertiary collecting ducts. Tertiary collecting ducts receive the distal tubules of looped nephrons. Thus, the fluid from all nephron types comingles as it passes through the medullary cone. The results of this study also show that the anatomical arrangement of medullary cones does not permit the output from one medullary cone to enter a second medullary cone. Thus, all the medullary cones function as parallel units. This anatomical organization of the avian kidney affects its ability to produce a urine hyperosmotic to the plasma.

Osmoregulatory responses of glucous-winged gulls (Larus glaucescens) to dehydration and hemorrhage

The effects of dehydration and hemorrhage on plasma ionic, osmotic, and antidiuretic hormone (arginine vasotocin) concentrations and of hemorrhage on salt gland secretion and glomerular filtration rate were evaluated in glaucous-winged gulls, Larus glaucescens. Dehydration for 24 h did not affect plasma ionic, osmotic or arginine vasotocin concentrations; 72 h dehydration significantly elevated plasma osmolality, plasma sodium and chloride concentrations, and plasma arginine vasotocin concentration, but did not affect plasma potassium concentration. Constant infusion of 0.8 mol.1-1 NaCl increased plasma arginine vasotocin concentration and produced salt gland secretion in seven gulls; four secreted well, while three secreted less well. Removal of 20% blood volume during saline infusion immediately reduced (P<0.001) salt gland secretion rate in all gulls. After bleeding, good secretors maintained glomerular filtration rate and urine flow rate; the poorer secretors increased glomerular filtration rate and became diuretic. Blood replacement returned salt gland secretion rate to the prebleeding level (P<0.05) without affecting salt gland secretions sodium concentration in gulls which secreted well, but did not restimulate salt gland secretion in gulls which secreted poorly. Reinfusion of blood had no effect on glomerular filtration rate. Bleeding and blood replacement did not affect plasma arginine vasotocin concentration.